| The appearance of the electrical tree in cable insulation is one of the typical characteristics of cable aging.Once the tree is initiated,driven by the electrical stress,it would develop gradually and finally penetrate the insulation,which causes the cable breakdown and seriously threatens the safety of the power system.Especially along with the continuous enhancement of the transmission voltage grade,the working field strength of insulation has greatly increased,and the probability of electrical tree initiated by defects in the insulation,such as conductive bump,has raised significantly.Therefore,it is important to explore the development characteristics of electrical tree in cable insulation and seek an effective method to evaluate the tree aging state of cable insulation.It is well known that partial discharge(PD)is an important feature of electrical treeing in cable insulation.So far,many scholars have carried out relevant researches on the characteristics of PD during electrical tree growth,but most of the work focused on the qualitative discussion of the correlation between the PD signal and the tree growth.Especially,there was a lack of systematic research on the characteristics of PD during the DC electrical tree growth.In addition,the study of charge transport under extra high field was relatively limited.Thus,it is impossible to give an exact answer to the question of whether there is a substantial difference in the mechanism of electrical treeing in insulating materials under AC and DC voltages.Moreover,it is impossible to use this difference to guide the detection and evaluation of PD during electrical tree growth in cable insulation.Space charge is a key parameter that affects the characteristics of electrical tree growth and PD under AC and DC voltages.In order to seek an effective method to quantitatively evaluate the tree aging state of cable insulation,this paper took cross-linked polyethylene(XLPE)used for AC cable(AC material),XLPE used for DC cable(DC material)and nano-Mg O/XLPE used for 320 k V DC cable as the research objects,and carried out the experiment of electrical tree and PD under AC and DC electric fields under needle-plate electrode,respectively.Besides,according to high-field conduction and space charge tests,the charge injection and migration behaviors under extra high electric field during treeing process were studied.On this basis,a gas-solid two-phase charge simulation model for electrical tree was developed to reveal the effect of space charge on PD during electrical tree growth under different temperatures.In the one-hour AC tree experiment,there were three typical stages in the development of electrical tree: rapid growth in branch shape,slow change in branch-pine shape,and stagnation in bush shape.For the same material,due to the stress fatigue of materials under high electrical field and the increasing movement of molecular chain under high temperature,the increase of electric field and temperature promoted the electrical tree growth and the PD level.Under the same experimental conditions,AC material was close to DC material in terms of electrical tree length,width and PD characteristics,while nano-Mg O/XLPE exhibited the best performance of AC tree resistance.In addition,as the electrical tree grew,the PD intensity in positive half cycle of voltage was greater than that in negative half cycle of voltage,and the maximum of PD was significantly higher than the average PD;but as the tree was in the insignificant growth period,the PD in the positive and negative half cycle of the voltage was symmetrical,and the maximum of PD was close to the average PD.Based on the PD signal during AC treeing,an Error Back Propagation(BP)neural network was established to achieve the quantitative assessment of AC tree aging state of insulation material.Under the experimental conditions,DC tree developed in a typical branch shape,which would turned and grew along the horizontal direction near the ground electrode when there was no breakdown channel of tree.Meanwhile,some tree channels had ability to self-heal.For the same material,the increase in temperature reduced the growth voltage of DC tree,accelerated the tree growth,and increased the PD intensity.In addition,the growth of tree along the electric field enlarged the amplitude of PD,while the extension of tree along the horizontal direction stabilized the PD.Under the same experimental conditions,nano-Mg O/XLPE exhibited the best performance of DC tree resistance,and the tree length,tree width and PD intensity in DC material under DC voltage were all smaller compared with AC material.According to the typical temporal information of PD signal,a Long Short-Term Memory(LSTM)network was established to realize the prediction of the PD data during treeing in multi-time steps,and the assessment of DC tree aging state of the insulation material was completed through BP neural network.In order to analyze the differences in PD characteristics during treeing under AC and DC voltages,the charge injection and migration behaviors under extra high electric field during treeing process were studied through high field conduction and space charge experiments on the basis of tree experiment.The results showed that from low to extra high electric field,the J-E curve of XLPE material corresponded to the modified space charge limited current theory,in which Schottky current was close to the experimental current at the field lower than 100 k V/mm,but the Schottky injection was no longer suitable for characterizing the charge injection process under extra high electric field.In addition,the nonlinear coefficient of the J-E curve was reduced under extra high electric field,and the injection of space charge in material was limited,which was related to the negative differential mobility of charge under extra high electric field.Based on this,a mobility-limited charge injection equation considering the negative differential characteristics of carrier mobility was proposed,and the validity of the equation was verified by space charge simulation,which provided a theoretical basis for space charge simulation in uneven electric field,such as needle-plate electrode.Based on the mobility-limited charge injection equation,the space charge simulation in gas-solid two-phase structure for electrical tree was carried out.The results showed that the homo-charge around the needle tip in solid insulation presented an “apple” shape,but due to the shielding effect of the homo-charge,the electric field inside the tree channel decreased,which weakened the PD intensity.In addition,the PD in tree channel under positive voltage developed in the form of positive charge packet,and the density and migration speed of charge packet gradually decreased.As temperature increased,the charge migration and diffusion in the solid insulation were accelerated,causing the increase in the electric field at the needle tip.Meanwhile,the higher temperature also increased the number and the migration speed of charged particles in the gas tree channel.As a result,the PD intensity occurred in tree channel was enhanced and the tree growth was accelerated.In addition,in the solid insulation,due to the wider distribution of negative charge in negative half cycle of the voltage,the shielding effect of negative charge on the needle electrode was more obvious.Thus,during the process of AC treeing,the electric field was higher and the PD intensity was greater in the positive half cycle of the voltage. |
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